Cationic electrodeposition coating compositions are widely used industrially as a primer for automobile body because they exhibit excellent rust resistance. Generally, cationic electrodeposition coating compositions are provided in the form such that a binder component which comprises a cationic resin (such as amine modified epoxy resin, for example) and a curing agent (also referred to as a cross-linking agent such as blocked polyisocyanate compound, for example) and a pigment paste containing pigment dispersed with a pigment dispersing resin, are dispersed in an aqueous medium.
A cationic electrodeposition coated film is obtained by using such a coating composition for a coating bath, in which electric current is passed between an object to be coated as cathode and a counter electrode to form a deposition coated film on a surface of the object to be coated, followed by heating the deposition coated film for curing it by cross-linking. As for the cationic electrodeposition coated film obtained in this manner, it is desired that the coated film is smooth and does not have surface discontinuity such as dimple and recess referred to as a crater or cratering, and that even if an intermediate coating and/or a finish coating are applied thereon, sufficient adhesive property therewith can be achieved and no adverse affect is exerted on smoothness, gloss and the like.
It is generally considered that a crater occurs from a nucleus which may be an organic compound of low surface tension contained in the coating composition, impurities contained in the pigment, dusts in the oven used in a baking process or oil or the like which is scattered from panel gaps of a car body. In particular, oil etc. may cause a dimple on a coated surface because it bumps at the time of being baked to cure. Such a problem of cratering can be solved by removing the pollution sources, however, it is substantially difficult to practice in industrial scale. Furthermore, in order to prevent occurrence of a crater, such methods had been conducted that ratio of pigment to resin in a coated film was increased and molecular weight of a basic resin was increased to suppress flow of a coated film at the time of baking to lower level. However, both of these methods had a drawback that smoothness of the coated film became poor because of increased viscosity.
A number of additives (called crater inhibiting agent, leveling agent or crater inhibiting agent, all of which are substantially the same meaning) for preventing occurrence of a crater for a cationic electrodeposition coating composition have been reported heretofore. For example, Japanese Patent Kokai Publication No. HEI 2-4826 discloses an additive using polyoxyalkylenepolyamine, and Japanese Patent Kokai Publication No. HEI 6-184471 discloses a fluorine-containing copolymer. However, when the additives are added in an amount that is enough to exhibit sufficient crater inhibiting effect, the coating composition may significantly increase in viscosity, or decrease in adhesive property with PVC sealer, intermediate coated film or finish coated film to be coated on the electrodeposition coated film.
Furthermore, Japanese Patent Kokai Publication No. 2000-7959 discloses an electrodeposition coating composition containing an acrylic resin having an amino group, an acid group and a hydroxyl group in one molecule as an crater inhibiting agent. However, the acrylic resin is inferior in compatibility with other resin components in the electrodeposition coating composition. Therefore, opportunity that the crater inhibiting agent can be added to an electrodeposition coating composition is limited to when a binder component including an amine modified epoxy resin or the like is emulsified, so that degree of freedom in preparation process of coating composition is reduced. For this reason, it is desired to develop an crater inhibiting agent without containing an acrylic resin, or a crater inhibiting method which can improves degree of freedom of preparation process of coating composition.